This invention relates to a process of recovering pure, concentrated ammonia from waste water which has preferably been freed to a large extent from dust, tar, oil, phenols and other organic substances and which consists particularly of condensates formed during the gasification or degasification of coal, or from aqueous liquors of similar composition, which in addition to ammonia contain acid constituents, such as carbon dioxide, hydrogen sulfide, small quantities of free and/or combined hydrocyanic acid and possibly residual quantities of a solvent used in a preceding dephenolation, which process comprises a stripping of gas water, de-acidification, and scrubbing.
Such solutions contain ammonia in concentrations between about 0.1 and 12 n, preferably between 0.3 and 3 n, carbon dioxide in concentrations between 0.1 and 12 n, preferably between 0.2 and 3 n, hydrogen sulfide in concentrations between 0.003 and 3 n, preferably between 0.003 and 0.3 n, and possibly hydrocyanic acid in an amount between 0.001 and 1 g/l.
In the solutions to be processed according to the invention, the weight ratio of the acid constituents (CO.sub.2 + H.sub.2 S) to the basic NH.sub.3 may be equal to or larger than unity or equal to or smaller than unity.
The process according to the invention enables a recovery of ammonia as an end product in a yield of virtually 100% and with a content of not more than 0.2% H.sub.2 O, 0.1% CO.sub.2, 10 ppm H.sub.2 S, and 10 ppm HCN. The resulting waste water is sufficiently pure for direct biological purification, if required.
The conditions of the present process are so selected that solutions having a surplus of acid constituents can be processed as well as solutions having a surplus of ammonia and that this can be accomplished without a substantial change of the conditions maintained during the several steps of the process. For an optimum control of the process, a simple change of the process sequence is sufficient. This change does not involve any problem as far as equipment is concerned and has not the least adverse influence on the purity of the end product, which is required to be of high purity for many purposes. If the weight ratio of acid constituents to ammonia is equal to or approximates unity, the selection of one or the other process sequences covered by the invention will be governed by economical considerations, e.g., in view of the concentrations or inert contents of the solutions to be processed.
Prior investignations have shown that the processing of solutions which contain ammonia and carbon dioxide and contain also free and/or combined hydrogen sulfide and possibly hydrocyanic acid rise to problems which are absent in the processing of solutions which contain only ammonia and carbon dioxide. See, e.g., H. Umbach: Die Entsauerung von Ammoniakwasser, Archiv fur bergbauliche Forschung, Year 3. No. 1, 1942, pages 49 et seq.).
From the published experimental data, it is apparent that ammonia in a particularly high purity as is required for numerous purposes cannot be recovered at all from pressurized gas water. Whereas in the process described there, the quantity of CO.sub.2 stripped off can be increased as desired by an increase in pressure so that an aqueous solution of ammonia which is virtually free from carbon dioxide is obtained in the sump of the column, the quantity of H.sub.2 S which is stripped off cannot be increased in this way and pressures under which all CO.sub.2 is stripped off result in an incomplete stripping off H.sub.2 S and vice versa so that ammonia meeting the above-mentioned specifications cannot be obtained.
A recovery of gaseous ammonia from strong ammonia water having a (CO.sub.2 + H.sub.2 S) : NH.sub.3 weight ratio &lt; 1 is described in German Patent 118,765. In that process, strong ammonia water must be diluted to an ammonia content of about 7-10% and must subsequently be de-acidified under pressures up to 15 kg/cm.sup.2 above atmospheric pressure. The partly de-acidified ammonia water is stripped off. The resulting exhaust gases contain CO.sub.2 and NH.sub.3 and must then be scrubbed in a scrubber with the original, H.sub.2 S-containing, strong ammonia water, which absorbs CO.sub.2, whereas pure ammonia is said to escape overhead. Any H.sub.2 S contained in these exhaust gases is removed in a subsequent scrubber. That process has the disadvantage that the pressure de-acidification stage is loaded in operation by the water which has been added at a high rate to the strong ammonia water, which has previously been concentrated. Another decisive disadvantage of the process resides in that ammonia having the above-mentioned purity cannot be produced at all in this manner. As has been mentioned above, it is essential to scrub the resulting exhaust gases, which contain NH.sub.3 and CO.sub.2, with the strong ammonia water which is to be processed. In view of experimental data published more than 20 years ago it can be proved clearly that the H.sub.2 S vapor pressure of the strong ammonia water used as a scrubbing liquor is so high that the escaping NH.sub.3 is bound to contain H.sub.2 S in an amount of the order of and above 100 ppm. Whereas the purity of the ammonia can be increased by a subsequent scrubbing with pure water, this involves a loss of ammonia and results either in a sewage which can be discarded only with difficulty or in loading of the process cycle with additional water if this loss in yield is to be compensated.
In accordance with German Patent 1,205,956, ammonia which is allegedly as pure as synthetic ammonia can be recovered from ammonia water which is similar in composition. In this case the (CO.sub.2 + H.sub.2 S) : NH.sub.3 weight ratio in the solutions concerned approximates unity. This process has also the disadvantage that water must be added to the previously concentrated gas water at a high rate of up to one-third of the raw water rate, and that the escaping vapors must not contain acid gases in excess of 33% by volume, in most cases only 15% by volume. Besides, that process is operated under atmospheric pressure in all stages.
In accordance with French Patent 1,106,881, carbon dioxide is removed from aqueous solutions of ammonia, such as gas water, in which the weight ratio of the acid constituents to ammonia approximately unity, although carbon dioxide is removed only in part and in no case entirely under the pressures of 2 - 6 kg/cm.sup.2 described there. That process is not directed to the production of ammonia of very high purity, and there is no need to remove virtually all hydrogen sulfide.
Australian Patent Application 206,296 relates also to the pressure decarbonization of strong ammonia water in which the weight ratio of the acid constituents to ammonia approximates unity, and also describes a partial de-acidification up to about 60%. More is not achieved by the measures described there. In that case too, the (CO.sub.2 + H.sub.2 S) : NH.sub.3 ratio also approximates unity and a complete removal of H.sub.2 S is not aimed at.
In accordance with U.S. Pat. No. 2,018,863, ammonia is also recovered from gas water. In that process, not all of the ammonia but only part thereof, preferably only about one-third, is to be recovered as free ammonia. A major part of the ammonia is combined with sulfuric acid which is supplied. The present process has nothing to do with that object of the prior invention. Besides, the prior process is apparently carried out only under atmospheric pressure.
In accordance with U.S. Pat. No. 2,162,838, hot gases produced by the distillation of coal are scrubbed with a weakly ammoniacal aqueous solution. The spent scrubbing liquor is initially distilled at a low temperature and in a vacuum. The effluent from the process cycle described there is a mixture which contains the compounds NH.sub.3, H.sub.2 S, and HCN approximately in the ratio in which they are contained in the raw gas. Residual CO.sub.2 may be removed from the scrubbing and cooling liquor by repeated distillation. The recovery of pure ammonia is not an object of that process. On the contrary, it is recommended to recover ammonia as ammonium sulfate, and any residual H.sub.2 S must be removed by an addition of iron oxide.
U.S. Pat. No. 3,556,721 relates to the purification of coke oven gases with formation of raw ammonia water. That prior process does not relate to the production of ammonia of very high purity, and the measures adopted in the present process for producing ammonia of very high purity cannot be derived from the prior process.
A number of other processes of recovering ammonia from gas water are known. Apparently because it was believed that a distillation of gas water and the like would not produce desired results, ammonia and the acid constituents are taken up in these processes by solid or liquid sorption agents, such as activated carbon, alkanolamines, sulfuric acid, and the like, and these sorption agents must subsequently be treated for a recovery of ammonia and may have to be regenerated.
The state of the art contains no teachings for a controlled and simple recovery of ammonia of very high purity from solutions having the stated concentrations and the started weight ratios of the acid constituents to ammonia.